3_FreeRTOS_Kernel_Priorities_Part3.pdf
- 1. Sherif Hammad Real Time Operating System Idle Task
- 2. 2 Agenda • Idle Task Call Back • Changing Tasks priorities • Scheduling Wrap up
- 3. 3 The Idle Task and the Idle Task Hook (Idle Task CallBack) • An Idle task is automatically created by the scheduler when vTaskStartScheduler() is called. • The idle task does very little more than sit in a loop • The idle task has the lowest possible priority (priority zero), to ensure it never prevents a higher priority application task from entering the Running state • Idle task is always pre-empted by higher priority tasks • idle hook (or idle callback) function—a function that is called automatically by the idle task once per iteration of the idle task loop • Common uses for the Idle task hook include: o Executing low priority, background, or continuous processing. o Measuring the amount of spare processing capacity. o Placing the processor into a low power mode • configUSE_IDLE_HOOK must be set to 1 within FreeRTOSConfig.h for the idle hook function to get called.
- 4. 4 Example 7: Idle Task CallBack
- 5. 5 Example 7: Idle Task CallBack How many times IdelTask is called between tasks calls?
- 6. 6 Changing task priorities; Example 8
- 7. 7 Changing task priorities; Example 8
- 8. 8 Changing task priorities; Example 8
- 9. 9 Changing task priorities; Example 8
- 10. 10 The Scheduling Algorithm—A Summary • Fixed Prioritized Pre-emptive Scheduling o Each task is assigned a priority. o Each task can exist in one of several states. o Only one task can exist in the Running state at any one time. o The scheduler always selects the highest priority Ready state task to enter the Running state. • Fixed Priority’ because each task is assigned a priority that is not altered by the kernel itself (only tasks can change priorities); • ‘Pre-emptive’ because a task entering the Ready state or having its priority altered will always pre-empt the Running state task, if the Running state task has a lower priority.
- 11. 11 The Scheduling Algorithm—A Summary